The present invention relates to digital images. More specifically, the present invention relates to the reconstruction of compressed images.
Image compression according to JPEG and certain other image compression algorithms is performed by converting images to luminance and chrominance channels and subsampling the chrominance channels (Cb and Cr) by an integer factor (usually 2 or 4). The luminance and subsampled chrominance channels are then compressed by a compression module suitable for grayscale image. The chrominance channels may be subsampled because the human visual system (HVS) has a reduced spatial sensitivity of chromatic sensation relative to luminance sensation by roughly a factor of two. Consequently, chromatic bleeding artifacts resulting from the sub-sampling are virtually unnoticeable in most color photographs that contain subsampled chrominance values.
However, chromatic bleeding artifacts are noticeable in color compound document images, which frequently contain sharp edges between different colors. For example, a color compound document might have text or other computer-generated shapes that are overlaid on regions having uniform color. The sharp edges occur at the transitions of the computer-generated images and the photographs.
The chromatic bleeding artifacts are noticeable if the reconstructed chrominance-channels are interpolated back to their original resolution by a simple technique such as linear interpolation. The chromatic bleeding artifacts are seen at boundaries between regions with saturated colors and white regions (or regions with bright colors). In document images, these boundaries are often between a white or bright background and color characters, lines and patches.
The chromatic bleeding artifacts are also noticeable in regions containing inverted text (e.g., white characters on a colored background). Due to the properties of the human visual system, the chromatic bleeding artifacts are visible on the brighter side of the edge.
The chromatic bleeding artifacts reduce image quality. Therefore, it would be desirable to reduce the chromatic bleeding artifacts, especially in color compound document images.